This invention relates to the preparation of cyclobutenoarenes. In a specific aspect, the invention relates to the purification of benzocyclobutenes prepared by the pyrolysis of o-methylbenzyl halides.
The four-membered ring of benzocyclobutenes is known to open at elevated temperature to form a very reactive diene which rapidly dimerizes and polymerizes. Molecules containing two or more benzocyclobutene groups are therefore useful as heat-curable thermosetting resins. Also, elastomers or thermoplastics containing benzocyclobutene substituents crosslink on heating. In order to economically prepare such useful benzocyclobutenefunctional resins or polymers, however, an economic method of generating the benzocyclobutene structure in pure form is needed. In the past, the benzocyclobutene structure has been synthesized most commonly by pyrolysis of substituted or unsubstituted o-methylbenzyl halides at temperatures above 650.degree. C. and pressures below 2 mm Hg.
Benzocyclobutenes produced by vacuum pyrolysis tend to be quite heavily contaminated with isomeric styrenes and the phenylacetylenes formed by dehydrogenation of the styrenes. Formation of styrene and phenylacetylene tends to be a particularly severe problem at pyrolysis tube temperatures of 800.degree. to 900.degree. C., at which the highest conversions of o-methylbenzyl halides to benzocyclobutene per pass are obtained.
Because of their similar boiling points, styrenes and phenylacetylenes are extremely difficult to separate from benzocyclobutenes by distillation. For example, benzocyclobutene boils at 146.degree.-148.degree. C. at atmospheric pressure, while the boiling points of styrene and phenylacetylene are 145.degree.-146.degree. C. and 142.degree.-44.degree. C., respectively. If the styrene and phenylacetylene are not removed from the benzocyclobutene, problems tend to occur in subsequent functionalization reactions for the preparation of precursors of bisbenzocyclobutene resins or benzocyclobutene-functional monomers.
For example, a standard method of benzocyclobutene functionalization involves initial bromination of the side chain. If the benzocyclobutene undergoing bromination contains styrene or phenylacetylene, these impurities tend to form di- or tetrabromides, respectively, under the bromination conditions. Such side-chain brominated materials tend to lose HBr at the high pot temperatures of the subsequent distillation procedure. The HBr damages the vacuum pump, and HBr elimination also yields monobromostyrenes, which boil at temperatures close to the bromobenzocyclobutene product and hence contaminate the distilled material.
It would thus be desirable to find new methods to recover benzocyclobutene from a reaction product mixture containing benzocyclobutene and similar-boiling by-products such as styrene and phenylacetylene.